1. Field of the Invention
This invention relates in general to method and apparatus for detecting and imaging one or more measuring points which have a defined signal progression, as for example, in an integrated circuit.
2. Description of the Prior Art
When testing integrated circuits to locate faulty circuits it is desired to compare the characteristic of the integrated circuit to the rated characteristic so as to locate faults in the circuit. For this purpose, a check must be performed to see whether specific signal progressions are present at one or more measuring points within the integrated circuit.
There are five methods for making checks that have been disclosed by the prior art. The method of the so-called "voltage coding" is described in "Scanning Electron Microscopy" 1975 part I, Proceedings of the Eighth Annual Scanning Electron Microscope Symposium, Chicago, IIT Research Institute, Pages 465-471. The "voltage coding" method forms the dynamic voltage distribution of an integrated circuit on a video monitor. The "voltage coding" method enables a chronological allocation of the logic states of the various components and is therefore particularly suited for a fast function check of integrated circuits. The most serious disadvantge of this method is that it has a low upper limit frequency.
U.S. Pat. No. 4,223,220 discloses the so-called "logic state mapping". In such "logic state mapping" method, the dynamic voltage distribution is imaged with the assistance of a stroboscope effect. This "logic state mapping" method supplies a chronological resolution which is higher by orders of magnitude as compared to the "voltage coding" method given the same voltage resolution. The "logic state mapping" method also simplifies recording since the imaging of the dynamic voltage distribution can be photographed directly from the photo picture screen of a scanning electron microscope. In the "voltage coding" method by contrast a recording is only possible with a tape storage or with photos of a video monitor.
A third method which is disclosed in Feuerbaum, Electron Beam Testing Methods and Applications, Scanning, Vol. 5, 1983 pages 14-24 which is referred to as a "waveform" measurement, it is possible to measure the chronological signal progression as a measuring point. By means of employing a sampling method, this method makes it possible to also record extremely fast signal progressions with high chronological resolution. However, the measurement is extremely lengthy requiring several minutes and is relatively complicated.
It is also known with the assistance of two further methods to locate the measuring points which carry the periodic signals of specific frequencies. In the first of these methods, described in J. P. Collin in Proceedings of Jornees d'Electronique 1983, "Testing Complex Integrated Circuits: A Challenge", edited by the Swiss Federal Institute of Technology, Lausanne, Switzerland, Pages 283-298, Title: Une Alternative Economique au Contraste Potentiel Stroboscopique: Le Traitment du Signal d'Electrons Secondaires d'un Microscope a Balayage", the location of defined frequencies at a measuring point is executed upon employment of a "lock-in" amplifier. A signal having the sought-for frequency is thereby filtered out from a voltage contrast signal acquired at a measuring point inside an integrated circuit and is filtered out with the assistance of the "lock-in" amplifier and the intensity of this signal is then imaged as a brightness modulation. This method, however, is extremely slow and has a very low upper limit frequency.
When the last of the above described methods is combined with a stroboscopic imaging method, then the bandwidth limitation can be overcome. This modified method is disclosed in H. D. Brust, F. Fox, E. Wolfgang, Frequency Mapping and Frequency Tracing: Two Novel Electron Beam Testing Methods, Vortrag auf der Microcircuit Enginering Konferenz in Berlin, September, 1984 and is referred to as "frequency mapping" method. The "frequency mapping" method similar to the method of J. P. Collin will only allow a check to be performed as to whether a signal having a defined frequency occurs at a measuring point. It provides no information as to what the actual signal progression looks like.
The described methods disclosed by the prior art are difficult to implement and only allow the checking of a very few or of even a single interconnections are greatly limited in terms of their working frequency range and, thus, checking of integrated circuits is frequently not possible under normal operating conditions. Also, the methods known in the prior art are very slow and presume that the comparison of the measured and anticipated signal progression is manually undertaken or, respectively, that given signals having the same basic frequency require that an independent decision be made as to whether the anticipated signal progression is in fact present.